The present invention relates to a new and improved method of, and apparatus for, the enciphering and deciphering of information or data.
Generally speaking, the method of the invention for the enciphering and deciphering of information, contemplates enciphering the plain information signal groups at the transmitter end of the system and which are separated from one another by separation signals and at the receiver end of the system performing an appropriate signal deciphering operation. At the transmitter end, in the absence of plain information signals, there are inserted between the information signal groups fill signals which are insignificant as to their information content.
The apparatus of the invention for enciphering and deciphering information comprises an enciphering device arranged at the transmitter end of the system, which enciphers the received plain text in the form of information signal groups, separated from one another by separation signals, through the use of key characters. A deciphering device is arranged at the receiver end of the system, which deciphers by means of key characters the received information. At the transmitter end, in the absence of plain text, there is inserted between the plain information signal groups fill signals which are insignificant as to their information content, these fill signals likewise being separated or divided by separation signals.
In the telecommunications art the information or data is frequently transmitted in a binary coded form, either as a continuous pulse train or in the form of pulse packets. A classical example of such data transmission is the CCITT-Code No. 2 used in teleprinters. With this code each character is represented by a combination of five binary pulses of the same length. During the start-stop operation there is incorporated a start step and a stop step before and after each of the pulse groups representing a character, in order to insure for synchronous operation of the transmitter and receiver apparatuses. The start step has the same length as an information or data step, while the stop step has a length amounting to 1.4-fold the length of the information step.
With the CCITT-Code No. 5, wherein each character is represented by a combination of eight binary pulses, the start step likewise has the same length as a data or information step, whereas the stop step has twice such length.
While taking into account these characteristics it is presently possible to design transmission systems which reliably synchronously operate, without having to alter the make-up of the character groups. In synchronous operating transmission systems of this type, of which for instance one has been described in Swiss Pat. No. 495,096 and the corresponding U.S. Pat. No. 3,702,900, there are introduced into the trasmission path during the time intervals where at the transmitter end of the system there is not present any plain text which is to be processed, fill characters which are meaningless as concerns data or information content. These fill characters contain at least the usual separation characters, i.e. start-stop steps, in order to maintain the transmitter in phase for as long as possible and to again bring into synchronism the transitter after a possible connection interruption.
The signal flow along the transmission path enables recognition of those time intervals during which there are transmitted information containing characters, since the intervals between the information or data signals can be readily recognized owing to the identical fill signals. Even if such information containing characters are enciphered third parties are capable of drawing conclusions regarding the activity taking place at the transmitter and receiver end of the system merely because of the message length.
An enciphered transmission of the fill signals also would not be suitable for overcoming this drawback. Since in the mentioned time intervals there are enciphered at the transmitter end always the same fill characters it is possible to break the code, even if at the present time there are known in this regard safe key generators.